The present invention relates to a displacement control mechanism for a variable displacement type compressor which is operable to adjust the pressure in a pressure control chamber by supplying refrigerant gas in a discharge-pressure region of the compressor into the pressure control chamber and releasing the refrigerant gas in the pressure control chamber to a suction-pressure region of the compressor, thereby controlling the displacement of the compressor.
In a variable displacement type compressor provided with a pressure control chamber having therein a swash plate whose inclination angle is variable, the inclination angle of the swash plate decreases with an increase of the pressure in the pressure control chamber. On the other hand, the inclination angle of the swash plate increases with a decrease of the pressure in the pressure control chamber. When the inclination angle of the swash plate decreases, the stroke of a piston decreases thereby to decrease the displacement of the compressor. When the inclination angle of the swash plate increases, the stroke of the piston increases thereby to increase the displacement of the compressor.
Since the refrigerant gas which is supplied to the pressure control chamber has been already compressed, the operating efficiency of the variable displacement type compressor deteriorates as the amount of refrigerant gas released from the pressure control chamber to the suction-pressure region of the compressor increases. Therefore, the cross-sectional area of a release passage through which the refrigerant gas is released from the pressure control chamber to the suction-pressure region should be small as much as possible in view of the operating efficiency with the result that a fixed throttle is provided in the release passage so as to decrease the cross-sectional area thereof.
If the compressor is left in a stopped state for a long time, the refrigerant gas is changed into a liquid state and the liquefied refrigerant is accumulated in the pressure control chamber. When the compressor is started in such a state, the liquefied refrigerant is not released rapidly to the suction-pressure region if the release passage has a fixed throttle with a small cross-sectional area. As a result, the liquefied refrigerant is vaporized in the pressure control chamber and the pressure in the pressure control chamber is increased excessively. Therefore, it takes a long time before the displacement of the compressor is increased to a desired level after the compressor is started.
A variable displacement type compressor with a displacement control mechanism is disclosed in Japanese Patent Application Publication NO. 2004-346880 to solve the above problem. The displacement control mechanism of this Publication has a first control valve which adjusts the cross-sectional area of a supply passage through which refrigerant gas is supplied from a discharge-pressure region to the pressure control chamber and a second control valve which adjusts the cross-sectional area of a release passage through which refrigerant gas is released from the pressure control chamber to the suction-pressure region. The release passage of the displacement control mechanism of the same Publication includes a first release passage having the second control valve therein and a second release passage interconnecting the pressure control chamber and the suction-pressure region directly without the second control valve.
The first control valve of the Publication is an electromagnetic control valve which is operable to adjust the degree of opening by changing the electromagnetic force. When the first control valve is in de-energized state, the degree of opening of the first control valve is maximum and the inclination angle of the swash plate is minimum, accordingly. This state corresponds to the minimum displacement operation of the compressor in which the displacement thereof is fixed at minimum. When the first control valve is in maximum energized state, the degree of opening thereof is minimum and the inclination angle of the swash plate is maximum, accordingly. When the first control valve is in an energized state that is smaller than the maximum energized state, the degree of opening thereof becomes smaller than the maximum and then the inclination angle of the swash plate is between the maximum and the minimum. This state corresponds to an intermediate displacement operation in which the displacement is not fixed.
The second control valve has a spool accommodated in a spool chamber and separating the spool chamber into a valve chamber and a back pressure chamber. The back pressure chamber communicates with a pressure region downstream of the first control valve and the valve chamber communicates with the pressure control chamber through a valve hole and also with the suction-pressure region of the compressor through a communication passage. The spool is urged by a spring toward the back pressure chamber, i.e., in the direction to increase the degree of opening of the valve hole.
When the compressor is started and the first control valve is closed, the pressure in the back pressure chamber of the second control valve becomes substantially the same as that in the pressure control chamber and the spool of the second control valve is moved by the spring so that the degree of opening of the second control valve becomes the maximum. Thus, the liquefied refrigerant in the pressure control chamber is rapidly released to the suction-pressure region, thereby reducing the time before the displacement is increased to a desired level after the variable displacement type compressor has been started. Even if the amount of blow-by gas passing through from a cylinder bore to the pressure control chamber increases after the liquefied refrigerant is discharged from the pressure control chamber, the blow-by gas is flowed out through the first and second release passages as long as the first control valve closes the supply passage.
When the supply passage is opened slightly by the first control valve, the pressure in the back pressure chamber becomes greater than that in the pressure control chamber, with the result that the spool moves against the spring so that the degree of opening of the second control valve becomes minimum that is not zero. Therefore, the second control valve functions in the same way as the fixed throttle thereby to prevent the deterioration of the operating efficiency caused by providing the displacement control mechanism.
In the second control valve in the aforementioned Publication, the spring force of the spring is often set small so that the spool of the second control valve can move quickly in the direction to minimize the degree of opening of the second control valve when the differential pressure between the back pressure chamber and the pressure control chamber is small. For example, in a clutchless variable displacement type compressor which is connected to a drive source without a clutch mechanism, since the first control valve is not energized when the compressor is started, the spool of the second control valve moves quickly in the direction to minimize the degree of opening of the second control valve by the increased discharge pressure. Since the liquefied refrigerant in the pressure control chamber is then stirred and the pressure in the pressure control chamber increases, the spool is urged in the direction to minimize the degree of opening of the second control valve by the pressure of the pressure control chamber, with the result that the degree of opening of the second control valve can not be maximized. Accordingly, the liquefied refrigerant is not discharged to the suction-pressure region quickly after a start-up of the compressor and it adversely takes a long time before the displacement of the compressor is increased to a desired level.
In a clutch variable displacement type compressor which is connected to a drive source through a clutch mechanism, when the first control valve is energized during the operation of the compressor with the degree of opening of the first control valve greater than the minimum, the spool of the second control valve moves quickly in the direction to minimize the degree of opening of the second control valve as the discharge pressure increases. When the high-pressure blow-by gas is then discharged to the pressure control chamber, the pressure in the pressure control chamber increases and the refrigerant gas in the pressure control chamber flows into the back pressure chamber through the supply passage. Accordingly, the spool is urged in the direction to minimize the degree of opening of the second control valve by the pressure in the back pressure chamber, so that the second control valve is unable to maximize the degree of its opening. Therefore, the second control valve become unable to adjust the discharge of refrigerant gas through the release passage, so that the adjustment of the swash plate to the desired inclination angle cannot be accomplished.
The present invention, which has been made in light of the above problems, is directed to providing a variable displacement type compressor with a displacement control mechanism permitting the second control valve to operate at such a timing that prevents the above-described deterioration of the operating efficiency of the compressor.